1. Field of the Invention
The present invention relates generally to mobile radio systems, and in particular to code division multiple access (CDMA) systems.
2. Description of the Prior Art
As a general rule, in CDMA systems, one objective is to improve performance, i.e. in particular to increase capacity and/or improve the quality of service.
One technique that is routinely employed is the power control technique, in particular the closed loop power control technique.
The objective of closed loop power control on a link between a base station and a mobile station is generally to maintain the signal-to-interference ratio (SIR) as close as possible to a target SIR. In the downlink direction, for example, i.e. the direction from a base station to a mobile station, the mobile station transmits to the base station power control commands intended to reduce the difference between the SIR estimated by the mobile station and the target SIR. The target SIR can be determined by another control loop for maintaining a constant quality of service. The second loop is generally referred to as the outer loop, as opposed to the previous loop, which is referred to as the inner loop.
As a general rule, to obtain better performance, the reaction time of the inner power control loop, i.e. the time-delay between the transmission of a signal by the base station and the application by that base station of a corresponding power control command, is advantageously as short as possible.
In systems such as the Universal Mobile Telecommunication System (UMTS), in which the information transmitted is structured in frames which are in turn structured in time slots, a power control command is generally transmitted in each time slot, is generally obtained from the signal received during the preceding time slot, and is generally applied to the signal transmitted during the next time slot.
In theory, this transmission scheme can define a nominal reaction time for the inner power control loop. In practice, because the clocks used in the base stations and the mobile stations to define the above time structure are not stable, and because the mobile stations are moving around, the reaction time can be greater than the nominal reaction time, with the risk of degraded performance. It is then necessary to provide adjustments to prevent degraded performance. Also, the adjustments associated with the stability of the clocks and the speed of the mobile station are merely examples and there are other possible causes of adjustment; for example, the clock of the mobile station and the clock of the base station may not have exactly the same frequency, even if they are very stable.
To determine the adjustments to be effected, the difference between times of reception and transmission by the mobile station can be observed in the mobile station. If that difference departs significantly from a nominal difference T0, an adjustment must be effected.
Another technique routinely employed in mobile radio systems, and in particular in CDMA systems, is the soft handover technique whereby a mobile station communicates simultaneously with several base stations and receive performance can be improved by appropriate techniques of processing and combining the various signals received from the various base stations by the mobile station (in particular using a Rake receiver).
For the UMTS, for example, the 3rd Generation Partnership Project (3GPP) document 3G TS 25.214 V3.1.1 provides for the difference between the times of reception and transmission by the mobile station to remain within limits of T0±ΔT0 where T0 is a nominal difference and T0±ΔT0 corresponds to a receive window around T0. ΔT0 is equal to 148 “chips”, for example (in CDMA systems, the term “chip” refers to the unit transmission period after spreading the spectrum of the signal to be transmitted).
An adjustment must also be effected to prevent the times of reception by the mobile station falling outside the receive window for at least one of the base stations with which the mobile station is communicating. Otherwise receive performance may be degraded and the corresponding connection may be lost.
When the various aims of the adjustments have been determined in this way, the problem arises of finding an optimum implementation, given the two types of equipment concerned, namely the mobile stations (referred to as user equipment (UE) in the UMTS), and the base station (referred to as “Node B” in the UMTS), or more generally the combination of the base stations and the units that control them (referred to as “radio network controllers” (RNC) in the UMTS), that combination being referred to as the “UMTS Terrestrial Radio Access Network” (UTRAN) in the UMTS. FIG. 1 outlines the general architecture of this kind of system.
Accordingly, in the current version of the above document, which is version 3G TS 25.214 V3.1.1 (1999-12), the UE is always authorized to adjust its transmit times, even if it is in the “soft handover” situation with base stations that do not have the same time reference (which situation is referred to as “soft handover normal”). If the difference between the UE receive and transmit times is outside the fixed limits, the UE can inform the UTRAN of this, and the latter can then adjust its transmit time.
In a previous version of the above document, the UE was authorized to adjust its transmit times only if it was not in the “soft handover normal” situation.
The reasons for this change to the document are set out in the document TSG-RAN Working Group 1 meeting #9-TSGR1 #9(99) i76—Dresden, Germany. A first reason stated, for which the UE must still be authorized to adjust its transmit time, is that in the “soft handover normal” situation this can otherwise lead to abandoning a link which is in fact the better quality link whereas, if the UE is authorized to change its transmit times, it can effect such changes in such a manner as to retain the better quality link and abandon another link of lower quality. A second reason stated is that, because the UE clock is generally much less stable than the network clock, the UE can effect as many changes as are necessary to align its clocks with the network clock and, in the “soft handover normal” situation, avoid requesting the network to effect more adjustments than necessary, causing it to consume more signaling resources than necessary.
It has been observed that the current version of the above document nevertheless has its drawbacks. In particular, in the “soft handover normal” situation, the mobile station runs the risk of modifying its transmit times in the opposite direction to that chosen by the network to modify its own transmit times, which is totally undesirable and can even run the risk of an effect opposite that required from such adjustments. Also, the risk is increased by the fact that the adjustment algorithms implemented in the UTRAN are not standardized and are therefore not known to the mobile stations a priori. In other wards, the essential drawback of this solution is a lack of communication between UE and UTRAN.
The previous version of the above document did not suffer from this drawback. On the other hand, it has also been noted that it had a different drawback, which was that the UE was never authorized to adjust its transmit times in the “soft handover normal” situation, although in some cases it could be beneficial if it were authorized to do so (in particular depending on the type of adjustment algorithm implemented in the UTRAN), or likewise if the LIE were still authorized to effect such adjustments outside the “soft normal handover” situation, whereas in some cases (in particular according to the type of adjustment algorithm implemented in the UTRAN), it could be beneficial for it not to be authorized to do this. In other words, the essential drawback of this solution is a lack of flexibility.
It has also been noted that the two solutions also have the drawback of inflexible or insufficient performance for another reason, which is that in situations where the UE is authorized to effect adjustments, it always effects them the same way, in a predetermined manner, and not in a manner that could be adapted to suit each situation (in particular according to the type of adjustment algorithm implemented in the UTRAN).
One object of the present invention is to avoid these drawbacks.